This invention relates to an improved process for the liquid phase chlorination of 1,3-butadiene to 3,4-dichlorobutene-1 and 1,4-dichlorobutene-2, which is characterized by high yield, low solvent:product ratio, good specificity, low operating temperature, and results in ready and economical removal of unreacted 1,3-butadiene and solvent for reuse in the system.
1,4-dichlorobutene-2 is a valuable intermediate in the manufacture of certain important polyamides, such as Nylon 66, while 3,4-dichlorobutene-1 is an equally valuable intermediate in the manufacture of chloroprene, which is the basic monomer in the manufacture of a class of important synthetic rubbers known as the neoprenes.
3,4-dichlorobutene-1 and 1,4-dichlorobutene-2 are prepared commercially from 1,3-butadiene in vapor phase chlorination processes utilizing either adiabatic reactors or tubular reactors. In both methods the free radical chlorination process requires the use of large excesses of butadiene which are recycled and in addition the processes are characterized by low yields (about 91.1%), nonspecificity, and high temperatures of the order of 225.degree.-300.degree. C.
It is also known to chlorinate 1,3-butadiene in the presence of halogenated solvents in the liquid phase, for example in systems catalyzed by soluble quaternary ammonium, pyridinium, phosphonium, and sulfonium chlorides, as described in British Patent No. 1,435,826, or in the presence of ferric chloride catalysts, as described in German Patent No. 2347194 to produce 3,4-dichlorobutene-1 and 1,4-dichlorobutene-2 and in a plug-flow reactor as described in U.S. Pat. No. 2,369,117. Despite the fact that liquid phase chlorination results in higher yields of the desired 3,4-dichlorobutene-1 and 1,4-dichlorobutene-2 products compared to vapor phase processes, liquid phase chlorination has not been commercially attractive. This is primarily due to the operating costs for energy and large investment required for equipment to separate solvent from the low boilers, dichlorobutene products, and high boilers in several distillation columns and the additional investment in equipment to handle the larger process streams.
An object of this invention is to provide a liquid phase chlorination process that allows a reduction in solvent:dichlorobutene ratio compared to prior art processes and simplifies recovery of the valuable dichlorobutene products and unreacted 1,3-butadiene, especially a process that precludes the necessity for secondary distillation facilities to remove large amounts of solvent from the dichlorobutenes and low and high boiling by-products. Also, because the process of this invention provides for low reaction temperatures the amount of by-products formed during the chlorination of 1,3-butadiene is reduced with a concomitant increase in the yield of the desired dichlorobutenes, significantly above 91%, yields usually obtained in vapor phase processes.